The present invention relates to a method for fabricating a semiconductor device, and more specifically to a process for forming a lower electrode of a cylindrical capacitor incorporated in a semiconductor device such as a DRAM (dynamic random access memory).
Japanese Patent Application Pre-examination Publication No. JP-A-09-331043 (an English abstract of which is available from the Japanese Patent Office and the content of the English abstract is incorporated by reference in its entirety into this application) proposes a new process for forming a lower electrode of a cylindrical capacitor.
According to this proposed process, after a hole is formed to penetrate through an insulating film having a planar upper surface, a polysilicon film (which is used for forming the lower electrode of the cylindrical capacitor) is deposited to cover the planar upper surface of the insulating film and an inner surface of the hole. Thereafter, a positive photoresist is deposited to cover the polysilicon film and to fill up the hole. Then, the positive photoresist is exposed with such an exposure energy that the positive photoresist above the planar upper surface of the insulating film is completely rendered dissoluble to a developer while the positive photoresist filled up in the hole is not rendered dissoluble to the developer because of a difference in film thickness between the positive photoresist above the planar upper surface of the insulating film and the positive photoresist filled up in the hole. Therefore, when the positive photoresist exposed as mentioned above is developed, the positive photoresist above the planar upper surface of the insulating film is removed, but the positive photoresist remains within the hole formed in the insulating film. Thereafter, the polysilicon film is etched back using the positive photoresist remaining in the hole, as a mask, with the result that the polysilicon film on the planar upper surface of the insulating film is removed, but the polysilicon film covering the inner surface of the hole remains. Succeedingly, the remaining positive photoresist is removed, and the insulating film is removed, so that the polysilicon film remains in the form of a cylinder, which constitutes the lower electrode of the cylindrical capacitor.
In the proposed process mentioned above, however, the margin of the exposure energy for causing the positive photoresist to effectively remain only within the hole is very small. Therefore, another problem has been encountered in that even if the exposure energy is strictly controlled, when a photoresist coating condition varies, no positive photoresist remains within the hole. In this case, when the polysilicon film is etched back, the polysilicon film on a bottom of the hole is removed because the polysilicon film on the bottom of the hole is not covered with the photoresist, with the result that the lower electrode of the cylindrical capacitor becomes defective. This is one of causes that lower the production yield of a semiconductor memory.